It is well known in the art of fuel delivery systems for aircraft gas turbine engines to use a fixed positive displacement pump, such as a vane or gear pump, to pressurize fuel for subsequent delivery to the engine. The fixed positive displacement pump provides a flow whose volume is a function of the speed at which the pump is rotating. The relation of the change in volumetric output for a change in speed is linear in nature.
The demand for fuel increases as the speed of the turbine increases, although when measured as a function of the percentage of pump output, demand for fuel is greatest at either low speeds (engine start) or at high speeds (take-off). Therefore, in order to provide the desired flow of fuel to the turbine during normal flight operation, the excess fuel output from the fixed positive displacement pump must be bypassed from the fuel control back to the input of the fixed positive displacement pump or to a fuel reservoir.
The positive displacement pump must be sized to ensure an excess flow capacity at all possible operating conditions. Therefore, the pump must be sized for either low speed start conditions, or high speed takeoff conditions.
The speed for greatest fuel demand is unique to each engine and is a function of the minimum starting speed. For engine applications where the pump has been sized, based on start speed, there will be an excess amount of fuel available at higher speeds.
Today's aircraft manufacturers are moving toward lower engine starting speeds, which tend to drive pump design requirements. As discussed above, sizing pumps for low speed condition generally results in large amounts of bypassed (unused) fuel at higher speed engine operating conditions. This bypassed fuel is continually recirculated and results in significant fuel heating.
With the latest fuel efficient engine designs, excessive fuel heating becomes a serious problem. The increase fuel temperature requires the addition of fuel/oil coolers. Air is also used to reduce fluid temperatures. These devices increase the cost, weight, and fuel burn of the engine.
It is typical in fuel supply systems for aircraft to control the flow of fuel to the engine through the use of a metering valve in conjunction with a pressure regulating valve.
Operation of the metering valve and the pressure regulating valve is based upon incompressible flow theory which states that flow through a valve is a function of the area of the valve opening multiplied by the square root of the product of the pressure drop across the valve multiplied by the specific gravity of the fluid. The pressure regulating valve controls pressure drop across the metering valve and compensates for temperature variations in the fuel, and therefore the flow though the metering valve can be precisely controlled by varying the area of the opening of the metering valve window.
As stated above the pump is sized to provide excess fuel flow for all engine operating conditions. The excess fuel flow is bypassed from the metering valve inlet, by the pressure regulating valve, back to the pump inlet. To achieve a desired increase in engine speed, an electronic controller will increase the area of the metering valve window to set a desired flow of fuel to the engine. As the metering valve window increases, the flow of fuel to the engine increases and the amount of fuel bypassed by the pressure regulating decreases. As the flow of fuel to the engine increases, the speed of the engine will increase which in turn drives the positive displacement pump at an increased speed. The increase in pump speed increases the flow of fuel which will cause a rise in the pressure differential across the metering valve. The pressure regulating valve will then bypass a portion of the excess fuel output from the positive displacement pump to maintain the desired pressure differential across the metering valve.
In addition to the fuel required by the engine, the pump also provides a fuel flow having a minimum pressure which is a function of the fuel delivery system hardware. The pressurized fuel is used as a working fluid to position valves within the fuel control. Therefore, the fuel must be maintained at sufficient pressure to position the valves (force margin) and furthermore must have sufficient pressure to actuate the valves within a required response time (slew rate).
To maintain the engine fuel at a minimum system pressure, a minimum pressurizing valve is positioned downstream of the fuel metering valve in the engine fuel supply path. The minimum pressurizing valve receives the engine fuel flow as an input and is mechanically biased to close at the desired minimum pressure. Therefore, the pressure of the engine fuel flow must be greater than the desired minimum pressure in order to force the minimum pressurizing valve open thus allowing the flow of fuel to the engine.
Higher system pressures increase internal leakage of fuel system components and reduce the volumetric efficiency of the pump thus also increasing pump size.